You should be able to send a PO Acknowledgment from the PO and that will communicate the cancellation. It would be under next steps and from there you can create the document.

There, you can fill out the form and communicate the cancellation.

If you do not see the PO Acknologment form as an option from next steps off the PO when you are in your inbox, I would suggest calling in and having us take a look at it. If the trading partner is advising you to send that back, you should have access to it.